Editing Meta-analysis (section)

===Approaches===
In general, two types of evidence can be distinguished when performing a meta-analysis: [[individual participant data]] (IPD), and aggregate data (AD).<ref>{{Cite journal |last1=Tierney |first1=Jayne F. |last2=Fisher |first2=David J. |last3=Burdett |first3=Sarah |last4=Stewart |first4=Lesley A. |last5=Parmar |first5=Mahesh K. B. |date=2020-01-31 |editor-last=Shapiro |editor-first=Steven D. |title=Comparison of aggregate and individual participant data approaches to meta-analysis of randomised trials: An observational study |journal=PLOS Medicine |language=en |volume=17 |issue=1 |pages=e1003019 |doi=10.1371/journal.pmed.1003019 |doi-access=free |issn=1549-1676 |pmc=6993967 |pmid=32004320}}</ref> The aggregate data can be direct or indirect.

AD is more commonly available (e.g. from the literature) and typically represents summary estimates such as odds ratios<ref>{{Cite journal |last1=Chang |first1=Bei-Hung |last2=Hoaglin |first2=David C. |date=2017 |title=Meta-Analysis of Odds Ratios: Current Good Practices |journal=Medical Care |language=en |volume=55 |issue=4 |pages=328–335 |doi=10.1097/MLR.0000000000000696 |issn=0025-7079 |pmc=5352535 |pmid=28169977}}</ref> or relative risks.<ref>{{Cite journal |last1=Bakbergenuly |first1=Ilyas |last2=Hoaglin |first2=David C. |last3=Kulinskaya |first3=Elena |date=2019 |title=Pitfalls of using the risk ratio in meta-analysis |journal=Research Synthesis Methods |language=en |volume=10 |issue=3 |pages=398–419 |doi=10.1002/jrsm.1347 |issn=1759-2879 |pmc=6767076 |pmid=30854785}}</ref> This can be directly synthesized across conceptually similar studies using several approaches. On the other hand, indirect aggregate data measures the effect of two treatments that were each compared against a similar control group in a meta-analysis. For example, if treatment A and treatment B were directly compared vs placebo in separate meta-analyses, we can use these two pooled results to get an estimate of the effects of A vs B in an indirect comparison as effect A vs Placebo minus effect B vs Placebo.

IPD evidence represents raw data as collected by the study centers. This distinction has raised the need for different meta-analytic methods when evidence synthesis is desired, and has led to the development of one-stage and two-stage methods.<ref>{{cite journal | vauthors = Debray TP, Moons KG, van Valkenhoef G, Efthimiou O, Hummel N, Groenwold RH, Reitsma JB | title = Get real in individual participant data (IPD) meta-analysis: a review of the methodology | journal = Research Synthesis Methods | volume = 6 | issue = 4 | pages = 293–309 | date = December 2015 | pmid = 26287812 | pmc = 5042043 | doi = 10.1002/jrsm.1160 }}</ref> In one-stage methods the IPD from all studies are modeled simultaneously whilst accounting for the clustering of participants within studies. Two-stage methods first compute summary statistics for AD from each study and then calculate overall statistics as a weighted average of the study statistics. By reducing IPD to AD, two-stage methods can also be applied when IPD is available; this makes them an appealing choice when performing a meta-analysis. Although it is conventionally believed that one-stage and two-stage methods yield similar results, recent studies have shown that they may occasionally lead to different conclusions.<ref name="pmid23585842">{{cite journal | vauthors = Debray TP, Moons KG, Abo-Zaid GM, Koffijberg H, Riley RD | title = Individual participant data meta-analysis for a binary outcome: one-stage or two-stage? | journal = PLOS ONE | volume = 8 | issue = 4 | pages = e60650 | year = 2013 | pmid = 23585842 | pmc = 3621872 | doi = 10.1371/journal.pone.0060650 | doi-access = free | bibcode = 2013PLoSO...860650D }}</ref><ref>{{cite journal | vauthors = Burke DL, Ensor J, Riley RD | title = Meta-analysis using individual participant data: one-stage and two-stage approaches, and why they may differ | journal = Statistics in Medicine | volume = 36 | issue = 5 | pages = 855–875 | date = February 2017 | pmid = 27747915 | pmc = 5297998 | doi = 10.1002/sim.7141 }}</ref>